In imaging devices with variable focus, very often an autofocus implementation and method is provided to automatically focus the lens system of the imaging system of the imaging device on a specific region of interest in the image, which is also called a salient region of the image. Normally, autofocus implementations in mobile devices are based on sharpness measurements in the salient region of the image.
Autofocus implementations are normally considered to be one of the major drawbacks of cameras in existing mobile devices due to the slow speed. Depending on the quality and resolution of the image sensor, autofocus can require up to several seconds. This rather large amount of time can negatively influence the user experience, as well as the image quality, in particular when the image subject is moving.
Normally, in mobile devices, when the user points the mobile device with the camera to a certain scene, the autofocus implementation is always active by trying to iteratively maximize the sharpness of the salient region within the image. The salient region or region of interest is either automatically determined on the basis of specific parameters in the device, or manually identified by the user. In order to decrease the computational complexity in the iterative process, some implementations compute the sharpness in the salient region on low resolution image frames. Generally, from the user perspective, it is desirable to improve the speed of the autofocus in an imaging device of a mobile device.
Generally, in recent prior art mobile devices, the salient region is automatically detected, for example by some techniques to detect the human face in order to automatically focus on this region, or are selected by the user manually through a specific user interface.